The present invention refers to a method for producing an optical preform with a POD cladding glass layer of fluorine-doped quartz glass comprising forming SiO2 particles by means of a plasma burner in the presence of fluorine, depositing said particles on an outer cylinder surface of a cylindrical substrate body having a longitudinal axis, and directly vitrifying said particles, said substrate body when viewed in the direction of the longitudinal axis thereof having a non-round cross-section including at least one surface section that extends between support points of an enveloping circle and which has either no curvature or a curvature which is different from the curvature of the enveloping circle.
The optical preform is directly drawn into an optical fiber or it serves as a rod-shaped or tubular preliminary product for fiber production. In radial direction, it has a non-homogeneous refractive index profile which is also defined by the fluorine doping of the cladding glass layer.
Cross sections differing from a circular form effect a change in the light conduction in optical components; to be more specific, light modes (so-called helix modes) are thereby hindered and changed, which permits a more efficient injection of pump light into the laser-active core in the case of laser applications.